Seismic acquisition in frigid areas has always been difficult and potentially dangerous. This is especially true when the seismic acquisition is in marine areas that are partially or completely covered by ice. When open or partially obstructed water is available in these regions, it is only available for a limited period of time.
Initially dynamite was used as an energy source but air guns and vibrators replaced dynamite as energy sources due to the negative environmental impact dynamite might have. Air guns are a well known seismic energy source; but require that many access holes be drilled so the air guns can be placed into the water column under the ice. This is not only a slow process; but severely limits the size of the air gun array. Limiting the size of the air gun array has the potential to degrade the final seismic image.
Vibrators when used on the surface of pack ice generate large amplitude events known as ice flexure waves which seriously obscure the desired seismic reflection data. Technical literature and patents have addressed improvements but do not completely correct the problems. Several proposals eliminate the generation mechanism by directly coupling the vibrator to the ocean bottom, for example, using bottom penetration means. Practical considerations of stilt-leg (supporting or connecting means for connecting the penetration means to a support vessel, etc.) weight and longitudinal strength would limit the water depth to which this solution could be employed and limit the weather conditions under which it could be operated.
Beyond the technical issues associated with seismic sources and receivers, operations in frigid weather on pack ice present very significant safety and logistic issues. Pack ice is not static. In point of fact it is continuously moving and reforming. In this process, leads, open water areas, fissures, ice ridges and crevasses are created. Because of the continuing changes in the surface conditions, it is important to be able to quickly locate and extract personnel. The use of helicopters in these conditions is typical.
The use of Autonomous Underwater Vehicles (“AUV units”) and Remotely Operated Vehicles (“ROV units”) is taught in U.S. Pat. Nos. 6,474,254, 6,975,560, 6,588,980 and EP publication 0154968. These patents address the use of ROV units and AUV units to deploy and collect receiver data; but only address half of the problem because they use conventional, open water air gun sources. GB 1,376,513 addresses both sources and receivers; but proposes to deploy the sources (explosive charges) and receivers (hydrophones) from a submarine. The use of a submarine would effectively limit the exploration to deeper waters and would limit the receivers to be marine streamer, hydrophone cables. U.S. Pat. No. 4,730,692 describes the use of an ROV based source; but only for shear wave generation and without addressing the need for receivers.
There is a continuing need for a seismic acquisition methodology that can safely provide high production rates when acquiring data from under ice-covered water resources and can accommodate state of the art seismic techniques such as 3D and high density sampling. Additionally the methodology needs to be applicable to both summer and winter acquisition so seismic data acquisition can occur on a year round basis. High production rates and year round access are especially important when time critical 4D acquisition is to be employed.